1. Field of the Invention:
The present invention relates generally to fluid velocity measuring devices and more specifically to a sensor assembly using a self-heated thermistor for sensing the velocity of an air stream.
2. Description of the Prior Art:
It is often desirable to measure the velocity of air flow through a duct, such as in an air conditioning system. A wide variety of velocity detection methods have been devised. Perhaps the most common method of detecting fluid velocities in general is by measuring a pressure differential across a shaped member having known properties.
When measuring air flow at relatively low levels, pressure differentials are not reliable because of the magnitudes of the quantities involved. It is known to use various methods wherein the velocity of air flow is detected indirectly by measuring the capacity of the air to transfer heat from a heated element.
It is known to heat a thermistor by using a separate heating element. The thermistor is heated slightly above the ambient temperature, and the cooling effect of moving air is reflected as a change in the resistance. Means are sometimes provided to compensate for temperature fluctuations in the moving air stream.
Detection methods presently used in the art generally have accuracy limitations, especially at low velocities. This may be caused by nonlinearity in the electrical components and turbulence in the fluid flow.